Kohji Tomisaka - Niigata University

Kohji Tomisaka
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Kohji Tomisaka
Niigata University

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Astrophysics (28)
Astrophysics of Galaxies (10)
Solar and Stellar Astrophysics (7)
Cosmology and Nongalactic Astrophysics (2)

Publications Authored By Kohji Tomisaka

Authors: Derek Ward-Thompson, Kate Pattle, Pierre Bastien, Ray S. Furuya, Woojin Kwon, Shih-Ping Lai, Keping Qiu, David Berry, Minho Choi, Simon Coudé, James Di Francesco, Thiem Hoang, Erica Franzmann, Per Friberg, Sarah F. Graves, Jane S. Greaves, Martin Houde, Doug Johnstone, Jason M. Kirk, Patrick M. Koch, Jungmi Kwon, Chang Won Lee, Di Li, Brenda C. Matthews, Joseph C. Mottram, Harriet Parsons, Andy Pon, Ramprasad Rao, Mark Rawlings, Hiroko Shinnaga, Sarah Sadavoy, Sven van Loo, Yusuke Aso, Do-Young Byun, Eswariah Chakali, Huei-Ru Chen, Mike C. -Y. Chen, Wen Ping Chen, Tao-Chung Ching, Jungyeon Cho, Antonio Chrysostomou, Eun Jung Chung, Yasuo Doi, Emily Drabek-Maunder, Stewart P. S. Eyres, Jason Fiege, Rachel K. Friesen, Gary Fuller, Tim Gledhill, Matt J. Griffin, Qilao Gu, Tetsuo Hasegawa, Jennifer Hatchell, Saeko S. Hayashi, Wayne Holland, Tsuyoshi Inoue, Shu-ichiro Inutsuka, Kazunari Iwasaki, Il-Gyo Jeong, Ji-hyun Kang, Miju Kang, Sung-ju Kang, Koji S. Kawabata, Francisca Kemper, Gwanjeong Kim, Jongsoo Kim, Kee-Tae Kim, Kyoung Hee Kim, Mi-Ryang Kim, Shinyoung Kim, Kevin M. Lacaille, Jeong-Eun Lee, Sang-Sung Lee, Dalei Li, Hua-bai Li, Hong-Li Liu, Junhao Liu, Sheng-Yuan Liu, Tie Liu, A-Ran Lyo, Steve Mairs, Masafumi Matsumura, Gerald H. Moriarty-Schieven, Fumitaka Nakamura, Hiroyuki Nakanishi, Nagayoshi Ohashi, Takashi Onaka, Nicolas Peretto, Tae-Soo Pyo, Lei Qian, Brendan Retter, John Richer, Andrew Rigby, Jean-François Robitaille, Giorgio Savini, Anna M. M. Scaife, Archana Soam, Motohide Tamura, Ya-Wen Tang, Kohji Tomisaka, Hongchi Wang, Jia-Wei Wang, Anthony P. Whitworth, Hsi-Wei Yen, Hyunju Yoo, Jinghua Yuan, Chuan-Peng Zhang, Guoyin Zhang, Jianjun Zhou, Lei Zhu, Philippe André, C. Darren Dowell, Sam Falle, Yusuke Tsukamoto

We present the first results from the B-fields In STar-forming Region Observations (BISTRO) survey, using the Sub-millimetre Common-User Bolometer Array 2 (SCUBA-2) camera, with its associated polarimeter (POL-2), on the James Clerk Maxwell Telescope (JCMT) in Hawaii. We discuss the survey's aims and objectives. We describe the rationale behind the survey, and the questions which the survey will aim to answer. Read More

We have observed the Class I protostar TMC-1A with Atacama Millimeter/submillimeter Array (ALMA) in 12CO and C18O (J=2-1), and 1.3-mm dust continuum emission. Continuum emission with a deconvolved size of 0. Read More

Filaments are considered to be basic structures and molecular clouds consist of filaments. Filaments are often observed as extending in the direction perpendicular to the interstellar magnetic field. The structure of filaments has been studied based on a magnetohydrostatic equilibrium model (Tomisaka 2014). Read More

We have constructed two types of analytical models for an isothermal filamentary cloud supported mainly by magnetic tension. The first one describes an isolated cloud while the second considers filamentary clouds spaced periodically. Both the models assume that the filamentary clouds are highly flattened. Read More

We report Atacama Large Millimeter/submillimeter Array (ALMA) cycle 0 observations of C$^{18}$O ($J=2-1$), SO ($J_N= 6_5-5_4$) and 1.3mm dust continuum toward L1527 IRS, a class 0 solar-type protostar surrounded by an infalling and rotating envelope. C$^{18}$O emission shows strong redshifted absorption against the bright continuum emission associated with L1527 IRS, strongly suggesting infall motions in the C$^{18}$O envelope. Read More

We have conducted ALMA observations in the 1.3 mm continuum and 12CO (2-1), C18O (2-1) and SO (5_6-4_5) lines toward L1489 IRS, a Class I protostar surrounded by a Keplerian disk and an infalling envelope. The Keplerian disk is clearly identified in the 12CO and C18O emission, and its outer radius (~700 AU) and mass (~0. Read More

Herschel observation has recently revealed that interstellar molecular clouds consist of many filaments. Polarization observations in optical and infrared wavelengths indicate that the magnetic field often runs perpendicular to the filament. In this paper, the magnetohydrostatic configuration of isothermal gas is studied, in which the thermal pressure and the Lorentz force are balanced against the self-gravity and the magnetic field is globally perpendicular to the axis of the filament. Read More

The configuration and evolution of the magnetic field in star-forming cores are investigated in order to directly compare simulations and observations. We prepare four different initial clouds having different magnetic field strengths and rotation rates, in which magnetic field lines are aligned/misaligned with the rotation axis. First, we calculate the evolution of such clouds from the prestellar stage until long after protostar formation. Read More

We investigate molecular evolution from a molecular cloud core to a first hydrostatic core in three spatial dimensions. We perform a radiation hydrodynamic simulation in order to trace fluid parcels, in which molecular evolution is investigated, using a gas-phase and grain-surface chemical reaction network. We derive spatial distributions of molecular abundances and column densities in the core harboring the first core. Read More

We report the first three-dimensional radiation magnetohydrodynamic (RMHD) simulations of protostellar collapse with and without Ohmic dissipation.We take into account many physical processes required to study star formation processes, including a realistic equation of state. We follow the evolution from molecular cloud cores until protostellar cores are formed with sufficiently high resolutions without introducing a sink particle. Read More

We measured polarized dust emission at 350um towards the high-mass star forming massive dense clump IRAS 20126+4104 using the SHARC II Polarimeter, SHARP, at the Caltech Submillimeter Observatory. Most of the observed magnetic field vectors agree well with magnetic field vectors obtained from a numerical simulation for the case when the global magnetic field lines are inclined with respect to the rotation axis of the dense clump. The results of the numerical simulation show that rotation plays an important role on the evolution of the massive dense clump and its magnetic field. Read More

Ultraluminous infrared galaxies (ULIRGs) with multiple ($\ge 3$) nuclei are frequently observed. It has been suggested that these nuclei are produced by multiple major mergers of galaxies. The expected rate of such mergers is, however, too low to reproduce the observed number of ULIRGs with multiple nuclei. Read More

Affiliations: 1National Astronomical Observatory of Japan, 2National Astronomical Observatory of Japan

A first core is a first hydrostatic object formed in the course of dynamical contraction of a molecular cloud core. Since the inflow pattern changes drastically before and after the first core formation, it is regarded as a milestone in the star formation process. In order to identify the first core from a mapping observation, the features expected for the first core are studied for CS rotation transitions at radio wavelengths. Read More

We studied the formation process of star clusters using high-resolution N-body/smoothed particle hydrodynamcs simulations of colliding galaxies. The total number of particles is 1.2x10^8 for our high resolution run. Read More

A first adiabatic core is a transient object formed in the early phase of star formation. The observation of a first core is believed to be difficult because of its short lifetime and low luminosity. On the basis of radiation hydrodynamic simulations, we propose a novel theoretical model of first cores, Exposed Long-lifetime First core (ELF). Read More

The observational expectation of polarization measurements of thermal dust radiation is investigated to find information on molecular outflows based on magnetohydrodynamical (MHD) and radiation transfer simulations. There are two major proposed models for the driving of molecular outflows: (1) molecular gas is accelerated by a magnetic pressure gradient or magnetocentrifugal wind mechanism before the magnetic field and molecular gas are decoupled, (2) the linear momentum of a highly collimated jet is transferred to the ambient molecular gas. In order to distinguish between these two models, it is crucial to observe the configuration of the magnetic field. Read More

We perform a three-dimensional nested-grid radiation magneto-hydrodynamics (RMHD) simulation with self-gravity to study the early phase of the low-mass star formation process from a rotating molecular cloud core to a first adiabatic core just before the second collapse begins. Radiation transfer is handled with the flux-limited diffusion approximation, operator-splitting and implicit time-integrator. In the RMHD simulation, the outer region of the first core attains a higher entropy and the size of first core is larger than that in the magnetohydrodynamics simulations with the barotropic approximation. Read More

Origin of the magnetic field ubiquitous in the Universe is studied based on the Biermann mechanism, which is expected to work in the non-barotropic region. We perform a series of two-dimensional MHD simulations of the first generation supernova remnant (SNR) expanding in the inhomogeneous interstellar matter (ISM) and study the Biermann mechanism working in the interior of the SNR. Especially, we pay attention to the relaxation process of electron and ion temperatures via the Coulomb interaction. Read More

Affiliations: 1National Astronomical Observatory of Japan, 2Chiba University, 3Ibaraki University, 4National Astronomical Observatory of Japan, 5Nagoya University, 6Nagoya University, 7Nagoya University, 8Nagoya University, 9Nagoya University, 10National Astronomical Observatory of Japan
Category: Astrophysics

A survey for the molecular clouds in the Galaxy with NANTEN mm telescope has discovered molecular loops in the Galactic center region. The loops show monotonic gradients of the line of sight velocity along the loops and the large velocity dispersions towards their foot points. It is suggested that these loops are explained in terms of the buoyant rise of magnetic loops due to the Parker instability. Read More

Recent observations suggest molecular line ratios in millimeter and submillimeter bands may be a good tool to reveal the long-standing question on the origin of energy sources in obscured active galaxies -- AGN and/or starburst. Observations of actual molecular medium show in general inhomogeneous structures as well as high-resolution hydrodynamic simulations do. In order for precise interpretation of emergent line emission from the inhomogeneous molecular gas to probe the dominant energy source of active galaxies, we study characteristic features of emergent intensities via three-dimensional non-LTE (non-local thermodynamic equilibrium) line transfer simulations. Read More

We investigated the evolution of interacting disk galaxies using high-resolution $N$-body/SPH simulations, taking into account the multiphase nature of the interstellar medium (ISM). In our high-resolution simulations, a large-scale starburst occurred naturally at the collision interface between two gas disks at the first encounter, resulting in the formation of star clusters. This is consistent with observations of interacting galaxies. Read More

We performed 3-dimensional N-body/SPH simulations to study how mass resolution and other model parameters such as the star formation efficiency parameter, C* and the threshold density, nth affect structures of the galactic gaseous/stellar disk in a static galactic potential. We employ 10^6 - 10^7 particles to resolve a cold and dense (T < 100 K & n_H > 100 cm^{-3}) phase. We found that structures of the ISM and the distribution of young stars are sensitive to the assumed nth. Read More

Affiliations: 1Astrophysical Research Center for the Structure and Evolution of the Cosmos, 2National Astronomical Observatory of Japan, 3National Astronomical Observatory of Japan
Category: Astrophysics

The unusual mushroom-shaped HI cloud, GW 123.4--1.5, is hundreds of parsecs in size but does not show any correlations to HI shells or chimney structures. Read More

Fragmentation and binary formation processes are studied using three-dimensional resistive MHD nested grid simulations. Starting with a Bonnor-Ebert isothermal cloud rotating in a uniform magnetic field, we calculate the cloud evolution from the molecular cloud core (n=10^4 cm^-3) to the stellar core (n \simeq 10^22 cm^-3). We calculated 147 models with different initial magnetic, rotational, and thermal energies, and the amplitudes of the non-axisymmetric perturbation. Read More

We explore millimeter line diagnostics of an obscuring molecular torus modeled by a hydrodynamic simulation with three-dimensional nonLTE radiative transfer calculations. Based on the results of high-resolution hydrodynamic simulation of the molecular torus around an AGN, we calculate intensities of HCN and HCO^{+} rotational lines as two representative high density tracers. The three-dimensional radiative transfer calculations shed light on a complicated excitation state in the inhomogeneous torus, even though a spatially uniform chemical structure is assumed. Read More

Developing a theory of low-mass star formation ($\sim 0.1$ to 3~M$_{\odot}$) remains one of the most elusive and important goals of theoretical astrophysics. The star-formation process is the outcome of the complex dynamics of interstellar gas involving non-linear interactions of turbulence, gravity, magnetic field and radiation. Read More

We studied the collapse of rotating molecular cloud cores with inclined magnetic fields, based on three-dimensional numerical simulations.The numerical simulations start from a rotating Bonnor-Ebert isothermal cloud in a uniform magnetic field. The magnetic field is initially taken to be inclined from the rotation axis. Read More

We estimate the polarized thermal dust emission from MHD simulations of protostellar collapse and outflow formation in order to investigate alignment of outflows with magnetic fields. The polarization maps indicate that alignment of an outflow with the magnetic field depends on the field strength inside the cloud core; the direction of the outflow, projected on the plane of the sky, is aligned preferentially with the mean polarization vector for a cloud core with a magnetic field strength of 80 microgauss, while it does not tend to be aligned for 50 microgauss as long as the 1000 AU scale is considered. The direction of the magnetic field at the cloud center is probed by the direction of the outflow. Read More

We present the evolution path of the first core in rotating molecular cloud core and the evolution during the second collapse phase using axisymmetric numerical calculations. The structure and evolution of the rotating first core is characterized by the angular momentum $J_{\rm core}$ and mass $M_{\rm core}$, both of which increase with time by accretion from the infalling envelope. We find the evolution path of it can be considered as a sequence of equilibrium solutions with a constant $J_{\rm core}/M_{\rm core}^2$. Read More

The evolution of supernova remnants (SNRs) is studied, with particular attention to the effect of magnetic fields with axisymmetric two-dimensional magnetohydrodynamical simulations. The evolution of magnetic SNRs is the same as non-magnetic ones in the adiabatic Sedov stage. After a thin shell is formed, the shell is driven by the pressure of the hot interior gas (bubble). Read More

We discuss evolution of the magnetic flux density and angular velocity in a molecular cloud core, on the basis of three-dimensional numerical simulations, in which a rotating magnetized cloud fragments and collapses to form a very dense optically thick core of > 5 times 10 ^10 cm^-3 . As the density increases towards the formation of the optically thick core, the magnetic flux density and angular velocity converge towards a single relationship between the two quantities. If the core is magnetically dominated its magnetic flux density approaches 1. Read More

Subsequent to Paper I, the evolution and fragmentation of a rotating magnetized cloud are studied with use of three-dimensional MHD nested-grid simulations. After the isothermal runaway collapse, an adiabatic gas forms a protostellar first core at the center of the cloud. When the isothermal gas is stable for fragmentation in a contracting disk, the adiabatic core often breaks into several fragments. Read More

The evolution of a gas shell, swept by the supernova remnant of a massive first generation star, is studied with H_2 and HD chemistry taken into account. When a first-generation star explodes as a supernova, H_2 and HD molecules are formed in the swept gas shell and effectively cool the gas shell to temperatures of 32 K - 154 K. If the supernova remnant can sweep to gather the ambient gas, the gas shell comes to be dominated by its self-gravity, and hence, is expected to fragment. Read More

We have performed three-dimensional, non-LTE radiative transfer calculations for 12CO and 13CO lines, applying them to our high-resolution hydrodynamic models of the `torus' around a SMBH in an active galactic nucleus. The hydrodynamic simulations reveal inhomogeneous and turbulent gas structure on a sub-pc scale in a circum-nuclear starburst region. Thick disks interlaced with filaments, clumps and holes are naturally formed due to the interplay among energy feedback from supernovae, self-gravity of the gas, galactic rotation, and radiative cooling. Read More

The collapse of slowly rotating molecular cloud cores threaded by magnetic fields is investigated by high-resolution numerical simulation. Outflow formation in the collapsing cloud cores is also followed. In the models examined, the cloud core and parent cloud rotate rigidly and are initially threaded by a uniform magnetic field. Read More

This is the first paper about the fragmentation and mass outflow in the molecular cloud by using three-dimensional MHD nested-grid simulations. The binary star formation process is studied paying particular attention to the fragmentation of a rotating magnetized molecular cloud. We assume an isothermal rotating and magnetized cylindrical cloud in hydrostatic balance. Read More

Authors: Kohji Tomisaka1
Affiliations: 1National Astronomical Observatory Japan
Category: Astrophysics

Collapse of the rotating magnetized molecular cloud core is studied with the axisymmetric magnetohydrodynamical (MHD) simulations. Due to the change of the equation of state of the interstellar gas, the molecular cloud cores experience several different phases as collapse proce eds. In the isothermal run-away collapse ($n \lesssim 10^{10}{\rm H_2 cm}^{-3}$), a pseudo-disk is formed and it continues to contract till the opaque core is fo rmed at the center. Read More

Dynamical contraction of a slowly-rotating magnetized cloud is studied using the magnetohydrodynamical (MHD) simulations. In the isothermal stage ($n \la n_{\rm A} \sim 10^{10}{\rm cm}^{-3}$), the cloud evolves similarly to that expected from Larson-Penston self-similar solution and experiences the run-away collapse. However, after the central density exceeds $\sim n_{\rm A}$, an accretion disk is formed around the adiabatic core. Read More

If the angular momentum of the molecular cloud core were conserved during the star formation process, a new-born star would rotate much faster than its fission speed. This constitutes the angular momentum problem of new-born stars. In this paper, the angular momentum transfer in the contraction of a rotating magnetized cloud is studied with axisymmetric MHD simulations. Read More

Authors: Kohji Tomisaka1
Affiliations: 1Niigata University
Category: Astrophysics

Dynamical collapses of magnetized molecular cloud cores are studied with magnetohydrodynamical simulations from the run-away collapse phase to the accretion phase. In the run-away collapse phase, a disk threaded by magnetic field lines is contracting due to its self-gravity and its evolution is well expressed by a self-similar solution. The central density increases greatly in a finite time scale and reaches a density at which an opaque core is formed at the center. Read More

The importance of the interstellar magnetic field is studied in relation to the evolutions of superbubbles with a three-dimensional (3D) numerical magnetohydrodynamical (MHD) simulation. A superbubble is a large supernova remnant driven by sequential supernova explosions in an OB association. Its evolution is affected by the density stratification in the galactic disk. Read More

Affiliations: 1University of Michigan, 2University of Michigan, 3University of Virginia, 4Niigata University
Category: Astrophysics

We present ASCA and ROSAT observations of SN 1986J covering the period 1991 August to 1996 January. From observations with the ROSAT HRI and PSPC, we find that the 0.5-2. Read More

Authors: Kohji Tomisaka1
Affiliations: 1Niigata Univ.
Category: Astrophysics

Accretion flow in a contracting magnetized isothermal cloud was studied using magnetohydrodynamical simulations and a nested grid technique. First, the interstellar magnetized cloud experiences a ``runaway collapse'' phase, in which the central density increases drastically within a finite time scale. Finally, it enters an accretion phase, in which inflowing matter accretes onto a central high-density disk or a new-born star. Read More

Fragmentation process in a cylindrical magnetized cloud is studied with the nested grid method. The nested grid scheme use 15 levels of grids with different spatial resolution overlaid subsequently, which enables us to trace the evolution from the molecular cloud density $\sim 100 {\rm cm}^{-3}$ to that of the protostellar disk $\sim 10^{10} {\rm cm} ^{-3}$ or more. Fluctuation with small amplitude grows by the gravita- tional instability. Read More

Authors: Kohji Tomisaka1
Affiliations: 1Niigata University, Niigata, Japan
Category: Astrophysics

Gravitational collapse of the cylindrical elongated cloud is studied by numerical magnetohydrodynamical simulations. In the infinitely long cloud in hydrostatic configuration, small perturbations grow by the gravitational instability. The most unstable mode indicated by a linear perturbation theory grows selectively even from a white noise. Read More

Reanalysis of Einstein IPC data and new observations from the GINGA LAC indicate the presence of extended X-ray emission (10-50 kpc) around the starburst galaxy M82. Here we model this emission by calculating numerical hydrodynamic simulations of the starburst event to much later times and larger scales than previously considered. For our models, we adopt a supernova rate of 0. Read More